A widely spread misconception. In fact, unless you smoke regulary (read: daily) the standard test will come back negative after 3-4 days. If you smoke a lot it will take several months though.
While being stoned is unacceptable in the cockpit, previous marijuana experiences should not pose a problem if he keeps quiet about it. Most would agree that marijuana is the milder drug compared to alcohol. It is not physically addictive and while it is possible to, like any psychoactive substance, develop a 'mental' addiction, I've never heard of anyone being unable to quit smoking weed, even after very long periods of daily usage.
Besides it takes about six hours to sober up and flashbacks are extremly rare, if they aren't just another myth. Everyone should of course exercise caution - but *having* smoked weed in the past can't be disqualifying for a pilot career. Now if it was smack...